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Cephalopoda

There are approximately 650 to 700 extant species of cephalopods in two subclasses
and five orders. Cephalopods are strictly marine and are found in all of the
world's oceans.

Cephalopoda is the most morphologically and behaviorally complex class in
phylum Mollusca. Cephalopoda means "head foot" and this group has the most complex brain of any invertebrate. Cephalopods are characterized by a completely merged head and
foot, with a ring of arms and/or tentacles surrounding the head. The arms, tentacles,
and funnel are all derivatives of the foot. Members of the order Nautiloidea
have more than 90 tentacles; members of orders Sepioidea and Teuthoidea have
eight arms and two tentacles; and members of orders Ocotopoidea and Vampyromorpha
have eight arms. The mantle surrounds the visceral sac and possesses strong
muscles required for contraction of the cavity and respiration. An opening in
the mantle cavity serves as an inhalant aperture, whereas the funnel serves
as the exhalent aperture. All cephalopods have one pair of unciliated ctenidia
within the mantle cavity, with the exception of Nautilus, which has two pairs
of ctenidia. The movement of water over the ctenidia is controlled by muscular
contractions of the funnel or mantle wall. An external shell is possessed only
by the Nautiloidea. Sepioids and teuthiods have reduced inner shells, while
ocotopoids and vampyromorphans lack shells altogether. A cephalopod is also
characterized by a horny beak secreted by the walls of the buccal cavity, and
a radula within the buccal cavity.

All cephalopods are carnivorous. The strong beak is at the entrance to the
buccal cavity , on the floor of which lies the radula. There are two pairs of
salivary glands , one of which may be poisonous. The digestive tract consists
of three parts:
esophagus , which may contain a crop;
stomach , which mashes food;
and
caecum , where most digestion and absorption occur. The posterior portion of the caecum contains a diverticulum
that serves as an ink gland, producing a suspension of melanin that can be expelled
through the mantle cavity.

Locomotion in cephalopods is accomplished mainly by jet propulsion. To close
its mantle completely, a squid fits two cartilaginous ridges on the mantle wall
into two cartilaginous grooves on the opposite funnel wall; contraction of circular
muscles around the mantle cavity then forces water out the funnel. The funnel
can be aimed, allowing the animal to change its direction. Locomotion in other
cephalopods can be accomplished by other means. Octopoids can use their arms
to "walk," and sepioids and teuthoids possess lateral fins that can
propel the animal.

Cephalopods are gonochoric. A female typically possesses a single oviduct.
A male produces spermatophores that it transfers to the female's genital pore
by means of a specialized arm or tentacle. In some species, the specialized
arm tip may be pinched off and left in the female's mantle cavity: this is known
as the hectocotylus arm. Mating in some cephalopods includes courtship rituals
that may consist of color changes, body movements, or combinations of both.
Cephalopods exhibit spiral cleavage and are protostomous, but they have no larval
stage: their development is direct. Octopods typically tend their eggs until
hatching. Most cephalopods are semelparous.

With the exception of Nautilus, cephalopods contain pigment-rich cells in the
epidermis surrounded by cells containing contractile fibers. These cells, called
chromatophores, are responsible for the ability of the cephalopods to change
color and patterns accurately and rapidly in response to danger or emotion.
Chromatophores may also be under hormonal control. When the contractile fibers
are stimulated, they contract and expose a larger amount of color.

Cephalopods possess well-developed nervous systems and complex sensory organs.
The ganglia are large and close to each other, forming a large brain. Certain
upper lobes within the brain serve as controls for memory and learning. Cephalopods
also possess ganglia elsewhere within the mantle cavity linked to the brain
by giant axons that are involved with muscular contraction. The eyes in Nautilus
are primitive, but in other cephalopods are highly developed and resemble vertebrate
eyes with a cornea, lens, retina, and iris. These eyes are capable of forming
images and distinguishing colors.

Cephalopods are of considerable economic importance to humans. Many species
of squid and octopus are eaten. Nautilus shells are often used decoratively,
and the internal shell of a cuttlefish, or cuttle bone, is sold in the pet trade
as a calcium source for birds. Giant cephalopods such as squid and octopuses
are also a great source of sea-monster folklore.

It is believed that cephalopods evolved from an ancient group of gastropods.
The fossil record of cephalopods is extensive, and more than 10,000 fossil species
have been named. The Ammonoidea arose during the late Paleozoic and grew to
large numbers during the Mesozoic. An ammonite had an external, coiled shell
similar to that of Nautilus. Ammonites were very successful -- scientists have
described 600 genera based on shell type - but became extinct at the end of
the Mesozoic. Belemnoids, which also appeared in the Mesozoic, had internal
shells. They are believed to be the precursors of modern-day squid and cuttlefish.
However, the ancestors of these Recent animals may have been living before the
belemnoids.

Contributors

Kristen Wheeler (author), Daphne G. Fautin (author).

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

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This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.